The present invention is concerned with plasticized polyvinyl chloride (PVC) compositions and articles made therefrom, and more particularly concerns those PVC compositions containing one or more materials which have been added to the compositions as thermal stabilizers.
PVC is a common commodity plastic, with global production exceeded only by polyethylene and polypropylene. It is cost-effective, highly versatile and is used in many applications in rigid, semi-flexible and flexible form. Rigid PVC is often used for ductwork, piping and the like, for example, in water, sewage and drainage piping, where chemical resistance is needed but not flexibility or pliability. Plasticized PVC, on the other hand, finds application in films, sheeting, wire and cable coverings, moldings, conveyor belting, toys and hose, in addition to serving as a leather substitute and as a fabric covering for upholstered furniture, automotive seating and other articles.
Broadly speaking, plasticizers are materials which are combined with polymers such as polyvinyl chloride (hereinafter, PVC) to impart flexibility, extensibility and workability or some combination of these attributes to the polymer, as needed for a particular end use. Frequently, a combination of primary and secondary plasticizers is used, with the secondary plasticizers not acting in and of themselves to impart the desired attributes to the PVC but serving to improve the effectiveness of the primary plasticizer(s) and optionally offering other characteristics to a PVC composition in which the materials are incorporated.
Historically, the majority of primary PVC plasticizers have been petroleum-derived phthalates and benzoate compounds, dioctyl phthalate and diisononyl phthalate being notable examples. However, such petroleum-derived plasticizers are frequently expensive to produce and use because of fluctuations in the pricing and availability of petroleum, and are increasingly likely to remain so as petroleum reserves are reduced and new supplies prove more costly and difficult to secure. Further, certain of the petroleum-derived phthalate plasticizers have raised concerns for their potential to disrupt human endocrine activity, and regulatory controls have been established in a number of countries to address these concerns.
As a consequence, efforts have been made in recent years by a number of parties to develop biobased phthalate replacement plasticizers. Unmodified plant/vegetable oils are largely incompatible with PVC resin, but certain modified derivatives of such oils, such as epoxidized soybean oil (ESO), are compatible with PVC resin and have been actively investigated for use as a lower cost, renewable source-based alternative to the petroleum-based plasticizers, both as primary and secondary plasticizers.
Carbohydrate-based alternatives have also been investigated. Carbohydrates represent a well-known, abundant class of natural raw materials, but generally have been too reactive/unstable to be used directly in many industrial chemical processes as renewable alternative feeds because of the presence of large numbers of hydroxyl and/or keto functionalities. Consequently, derivatives of carbohydrates with reduced functionality and improved stability have been developed and investigated for a variety of uses, including for use as plasticizers for PVC or as a feedstock useful for making materials useful as plasticizers. An example is the Polysorb® ID 37 isosorbide diester plasticizer product being offered by Roquette Freres as a suitable biobased phthalate replacement plasticizer.
One of the carbohydrate-derived materials considered for use in making a biobased, phthalate alternative plasticizer for PVC is furan-2,5-dicarboxylic acid, more commonly referred to in the literature as FDCA. FDCA has been looked at primarily as a biobased alternative for terephthalic acid, which is currently produced on a multimillion ton scale from petrochemical feedstocks, but with FDCA's structural similarity to phthalates in general some work has been done also to evaluate the FDCA diesters as potential phthalate replacements in flexible PVC compositions.
The earliest publication known to Applicants describing the use of FDCA-based diesters as plasticizers for PVC is a 1994 paper by Sanderson et al. (Sanderson et al., “Synthesis and Evaluation of Dialkyl Furan-2,5-Dicarboxylates as Plasticizers for PVC”, Journal of Applied Polymer Science, vol. 53, pp 1785-1793 (1994)), wherein the synthesis of the 2-ethylhexyl, 2-octyl, n-hexyl, n-butyl esters from furfural was initially accomplished via a six-step synthesis. The desired FDCA diesters were then made by transesterification of the dimethyl ester of FDCA. Small samples of plasticized PVC were obtained by compression molding of PVC/plasticizer blends, and subsequently analyzed by dynamic mechanical thermal analysis (DMTA). The performance of the FDCA diesters as plasticizers was compared to that of di-2-ethylhexyl phthalate (DEHP, also known as DOP). While the plasticizing performance of the 2-octyl-FDCA ester was very similar to the results seen with the known phthalate plasticizer, and while improvements in compatibility were noted, the authors did observe discoloration of the FDCA diester-plasticized PVC compositions over time on exposure to light and air.
More recently, Grass et al, in published applications WO 2011/023491 and WO 2011/023590, described the synthesis and plasticizing performance of FDCA diesters from isononyl and isodecyl alcohols. Both diesters were found to have beneficial properties and advantages compared to the corresponding phthalate esters, and are described as useful alone or in combination with other plasticizers in “plastics compositions, adhesives, sealants, lacquers, paints, plastisols, artificial leathers, floor coverings, floor coverings, underbody coatings, coated fabrics, wallpapers, or inks”, as well as “in profiles, seals, food packages, films, toys, medicinal items, roofing materials . . . cables, and wire coatings . . . food packaging . . . bags and tubing materials for infusions, dialysis, and drainage.” Surprisingly, however, while the earlier work by Sanderson et al. is expressly acknowledged, neither of these applications mentions the discoloration observed by Sanderson et al, describes the presence or absence of color in the plasticized PVC compositions made or in the articles produced therefrom, or undertakes or reports any measurements related to color.